Thermoplastic elastomer (TPE) materials include those which generally have properties and performance similar to vulcanized rubber at service temperatures, yet can be processed and fabricated by methods commonly used to process thermoplastics. Thermoplastic elastomers can generally be reprocessed since they contain a thermoplastic component.
Thermoplastic elastomers based on blends of ethylene-propylene elastomeric polymers or ethylene, .alpha.-olefin, diene elastomeric polymers with semi-crystalline or crystalline polymers are known. Typically such compounds are made by dynamically curing the blend of rubber and resin. Shaped articles formed from the composition have generally elastomeric properties without the need for further cure.
In the past, such ethylene, alpha-olefin or ethylene, alpha-olefin, diene monomer elastomeric polymer/semi-crystalline or crystalline polyolefin type TPEs have found many uses. However, in some instances application has been limited by the fact that the dienes generally commercially available for making the ethylene, alpha-olefin, diene monomer elastomeric polymer portion of the thermoplastic elastomer do not yield a generally fully cured elastomer phase when peroxide is the curative. For purposes of this application, the term fully cured will mean any cure state above about 95 percent. The mechanism and means of determining cure state will be discussed further in this application. While a substantially full cure for previously available TPEs could be obtained by utilizing various non-peroxide curative mechanisms, peroxide cures of these or similar TPEs are generally not full cures because the amount of a peroxide compound necessary, combined with processing temperatures and shear (necessary for fabricating articles from the TPE), generally causes undesirable side effects in the crystalline or semi-crystalline polyolefin component of the TIE. These side effects may include, for example, cross linking of polyethylenes. In polypropylenes, a more common blend partner in TPEs, such side effects include chain scission of the polypropylene, resulting in an undesirable lowering of molecular weight. Therefore, a manufacturer or processor of thermoplastic elastomers walks a narrow line, on one side of the line lies the desirable more fully cured elastomer, but a degraded or property diminished crystalline or semi-crystalline polyolefin portion; and on the other side of the line lies a rubber portion with lower cure and a crystalline polyolefin portion that maintains substantially all of its original properties.
Many known thermoplastic elastomers utilize ethylene, alpha-olefin, diene monomer elastomeric polymers with varying amounts of a diene monomer generally selected from the group consisting of 5-ethylidene-2-norbornene, 1,4-hexadiene, 1,6 octadiene, 5-methyl-1,4 hexadiene, and 3,7-dimethyl-1,6-octadiene. Generally ethylene, alpha-olefin, diene monomer elastomeric polymers incorporating these types of dienes, when used in TPE compositions with peroxide curatives, suffer from the above discussed trade-off of increasing cure versus maintenance of the crystalline or semi-crystalline polyolefin properties.
Substantially fully cured TPEs are available utilizing other curative systems, for instance, phenolic type cures; an example would be Santoprene.RTM. (Monsanto Company) rubber. The general advantage of any fully cured or substantially fully cured TPE material is that its mechanical properties will generally be more desirable than a TPE with a lower degree of cure of the elastomer portion. However, the most important mechanical property for certain applications, resistance to compression set, will improve, generally with greater cure state. In TPEs having an ethylene, alpha-olefin, diene monomer elastomeric polymer cured less than about 90 percent, compression set is generally unacceptably high for many applications, especially at elevated temperatures.
It would be desirable, therefore, to have a commercial TPE available which allows an increased cure state, while having a minimum diminution of the crystalline or semi-crystalline polyolefin properties, as well as improved compression set resistance when compared to either partially cured peroxide, or substantially fully cured phenolic materials.